1. The Field of the Invention
This invention relates to a method and to composition. More specifically, this invention addresses itself to resolution of problems associated with the oxidation of dienic polymers and articles prepared from such materials. This invention also encompasses dienic polymer compositions which are resistant to oxidative degradation.
2. Description of the Prior Art
Polymers have in the past and continue to provide an attractive substitute for other more traditional types of structural materials (e.g. wood and metals) because of relatively inexpensive materials and fabrication costs. As polymers continue to find new applications in, for example, the fabrication of automotive parts and building materials, they must also become more durable and capable of withstanding prolonged exposure to a variety of degradative forces. Degradation of polymers can be caused by exposure to light, heat and/or air. Such degradation is usually manifest by either a partial or total loss of structural integrity, changes in light transmission properties, changes in color, loss or reduction in flexibility and resiliency or any combination of the above phenomena. Those attempting to avoid polymer degradation have generally selected from among three possible approaches: (a) elimination or reduction of the degradative forces; (b) isolation of the sensitive polymer material from the degradative forces; or (c) modification of the polymer composition to enhance resistance to one or more of the degradative forces. This latter approach is generally preferable since it does not require engineering or structural changes in the polymer products environment.
There are a variety of additives which have been disclosed as suitable for enhancing the resistance of polymers to one or more of the degradative forces discussed hereinabove. These additives (hereinafter referred to as "stabilizers") can be physically combined with or engrafted upon the environmentally sensitive polymer, thereby prolonging its useful life in the hostile degradative environment. Stabilizers are available which can enhance the polymers resistance to more than one of the degradative forces and, conversely, a stabilizer which is effective for prevention of, for example, oxidative degradation may have little if any effect upon the polymers resistance to other degradative agents. Thus, it is not uncommon for polymers to contain a variety of stabilizer materials, each being present for the prevention of a particular degradative reaction. One of the more difficult to control of the degradative forces is the exposure of the polymer to oxidation by the air. The impact of such exposure will, of course, vary depending upon the temperature prevailing during such exposure and the presence and/or absence of other agents either within or in contact with the polymeric material which can accelerate such degradation. Ordinarily, the effects of oxidation manifest themselves only after a prolonged and/or repeated exposure. The exposure of polymers to the degradative forces of oxidation can result in a variety of changes in such materials depending upon the degree of saturation of a polymer backbone and the various substituents pendant therefrom.
In the case of dienic polymers, the effects of oxidative degradation may be different depending upon whether or not the polymer is vulcanized. Ordinarily, unvulcanized dienic polymers will manifest oxidative degradation by becoming increasingly pliable, and in some extreme cases, tacky. By way of comparison, vulcanized dienic polymers will manifest oxidative degradation by becoming increasingly hard and brittle. As will be appreciated, such physical changes in polymer properties, and articles prepared therefrom, will adversely alter the structure of the polymeric article to the extent that failure of the article may occur.
Unfortunately the problems associated with oxidation of polymers are especially acute in dienic polymers. The reasons for such sensitivity is the presence of unsaturated linkages in the polymer backbone or in side-chains. Such unsaturation is particularly vulnerable to oxidation and such oxidation can be accelerated by various forms of radiant energy (e.g. heat and/or ultraviolet lights). Many different classes of materials have been previously disclosed as suitable for use in the stabilization of polymeric materials against the degradation from the forces discussed herein. Such stabilizers are, however, only generally effective for the prevention of one form of degradative reaction and thus, it is necessary to use combinations of stabilizers to impart the desired protection to polymeric materials where exposure to several degradative forces and/or agents is contemplated.
The stabilization of dienic polymers against oxidation can be achieved by the incorporation within such polymers of naphthofuranyl naphthyl compounds (U.S. Pat. No. 3,940,364); aliphatic esters of carboxymethene and carboxyethenethio succinic acid (U.S. Pat. No. 3,909,493); hydroxyphenyl alkyleneyl isocyanurate/thiophosphite mixtures (U.S. Pat. No. 3,909,491); and benzofuranylphenols (U.S. Pat. No. 4,054,551). The above listing is by no means exhaustive of the various types of compounds and mixtures of compounds which have been previously used in the stabilization of dienic polymers against the forces of oxidative degradation. In a number of instances, the materials useful in the stabilization of dienic polymers against oxidative degradation are also effective in the stabilization of other polyolefins (e.g. see for example previously referenced Patents '493 and '491).
As noted above, such oxidation can be accelerated where the polymer is exposed to radiant energy. Thus, it may be desirable to use combinations of stabilizers in order to eliminate or reduce the contributing causes to such degradation. Some stabilizers, such as Goodrite 3114, [1,3,5-tris(3'5'-di-t-butyl-4'-hydroxy-benzyl)-s-triazine-2,4,6(1H,3H,5H)- trione]--available from The B. F. Goodrich Chemical Company, are highly effective for the stabilization of polyolefins against the degradative forces of oxidation and ultraviolet light. Other commercially available stabilizer materials, such as Tinuvin 770 [bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate]-available from Ciba-Geigy, are of limited effectiveness in preventing oxidation of polymers, although being highly effective in prevention of UV degradation. This has also been generally true of other hindred amines.
The effectiveness of a stabilizer material is measured not only in absolute terms, but also requires evaluation of its compatibility with the stabilized host polymer and its effectiveness at low concentrations. As will be appreciated, the addition of high concentrations of stabilizer compounds to polymeric materials can adversely impact the processing of such materials and the physical properties of the articles prepared therefrom. Thus, stabilizer effectiveness, and its commercial acceptance, is based not solely upon its capabilities regarding the prevention of certain degradative reactions but also its ability to do so at low concentrations without adversely altering the polymers physical properties and its ability to achieve such beneficial results in the presence of other stabilizer materials. All of the above factors must be taken into consideration in evaluating the efficiency of a stabilizer, and as might be expected, the larger the number of factors to be considered in such evaluation, the more difficult for a stabilizer to achieve commercial acceptance. Because of such changing and ever demanding requirements, there is a continuing need for development of new stabilizer systems which can satisfy the technical specifications of the compounder, are cheap to synthesize and are highly compatible with the host polymer and other additives used in conjunction therewith.